1. Field of the Invention
The present invention relates to improved blood contact surfaces for use in apparatus such as in artificial blood vessels and other implantable appliances, and methods for synthesizing the improved blood contact surfaces in vitro.
2. Description of Related Art
A common surgical practice in the treatment of occlusive atherosclerotic disease of peripheral arteries is to transplant a section of living vein, taken from the same patient, as a bypass around the occluded region of artery. Approximately one fourth of all patients requiring peripheral arterial bypass, however, have saphenous veins unsuitable for use because of varicosities, multibranching, or inadequate diameter. These patients then require some alternative material with which the bypass can be effected. In addition, the transplanted vein itself is often susceptible to the atherosclerotic disease process. This atherosclerotic process is similar to that of the artery but the vein is afflicted at an accelerated rate, frequently causing vein graft failure and necessitating an additional bypass. For these reasons, a need exists for a vascular substitute that would perform at least comparably to the autologous saphenous vein in the small diameter application.
In addition to autologous living tissues transferred from one location to another within the same individual, other biological materials have been used in this application. These include treated donor allogeneic or xenogeneic tissue components, such as processed human umbilical vein, cryopreserved allogenic vein, or processed xenogeneic artery. Although living vessel transplants from one patient to another are employed, the foreign vessel typically dies following transplant.
In addition to tissues of biological origin, synthetic materials are also commonly used to replace blood vessels. Synthetic vascular grafts have been used successfully since the 1950's to replace large vessels such as the aorta or iliac arteries. The principal synthetics used for grafting include polyethylene terephthalate and expanded polytetrafluoroethylene (ePTFE), although other materials utilized include polypropylene, polyurethane, and polydimethyl siloxane. Both porous and nonporous constructions of these polymers have been used.
Although both biologic and synthetic materials have been used with good success in some applications, such as bypasses from the femoral artery to the segment of the popliteal artery above the knee, both synthetic and biologic materials have been shown to thrombose much more frequently than the autologous saphenous vein when used for small diameter bypasses such as the coronary or below knee arteries. This performance difference limits the usefulness of such vascular replacements in these more demanding applications.
Natural blood contact surfaces, such as those found within blood vessels, have inherent mechanisms to prevent thrombosis during normal passage of blood along the surface. In the case of a mammalian artery, the immediate blood contact surface consists of a layer of endothelial cells that is nonthrombogenic. Immediately external to the endothelial cell layer is the remainder of the intima: a subendothelial matrix layer consisting of basement membrane and an underlying layer of glycoprotein-bearing extracellular matrix, and the internal elastic lamina. Surrounding the intima layer is the multilaminate media structure containing smooth muscle cells and elastin, and surrounding this, the most external layer, comprised of fibroblasts and connective tissue, the adventitia. As is explained in greater detail below, it is generally accepted that the subendothelial layer and media are thrombogenic in nature in order to maintain hemostasis when the vascular system is injured. See for example: J. A. Madri et al., "The Collagenous Components of the Subendothelium,"Lab. Invest. 43:303-15 (1980); and T. Matsuda et al., "A Hybrid Artificial Vascular Graft Based Upon an Organ Reconstruction Model:. Significance and Design Criteria of an Artificial Basement Membrane,"ASAIO Transactions 34:640-43 (1988).
Knowledge of the detailed mechanisms by which natural vessels maintain patency has been very limited. The predominate theory relating to the effective function of normal vasculature is based upon the necessity of a healthy, intact, vascular endothelium to serve as the blood interface. When the endothelial lining is removed, thrombosis of the vessel is a frequent occurrence. Supporting this theory, in part, are numerous experiments showing the endothelium to have a unique clot inhibiting effect on blood with which it is in direct contact. Endothelial cells have been further shown to synthesize or bind a number of substances with coagulation inhibiting or fibrinolytic function including heparan sulfate/antithrombin III, dermatan sulfate/heparin cofactor II, thrombomodulin/protein C/protein S, prostacyclin and tissue-type plasminogen activator. Thus, based on these experimental observations, and the blood contacting location of the endothelium in the vascular system, it has been widely accepted that the endothelium, is primarily, if not wholly, responsible for the antithrombotic behavior of blood vessels. See for example, R. G. Petty et al., "Endothelium-the Axis of Vascular Health and Disease," J. Royal Coll. Phys. London 23:92-102 (1989); and R. E. Scharf et al., "Thrombosis and Atherosclerosis: Regulatory Role of Interactions Among Blood Components and Endothelium," Blut 51:31-44 (1987).
In support of this concept, numerous experiments investigating the reactivity of blood to non-endothelialized vessels have been reported suggesting that the subendothelium and underlying structures are thrombogenic, particularly with respect to platelet adhesion and degranulation. These observations come from both in vivo and in vitro thrombosis assays.
In addition to the immediate subendothelial matrix layer, smooth muscle cells in the deeper media layer are generally considered to be thrombogenic as well. See for example, S. M. Schwartz et al., "The Aortic Intima: II. Repair of the Aortic Lining After Mechanical Denudation," Am. J. Pathol. 81:5-42 (1975); J. J. Zwaginga et a., "Thrombogenicity of Vascular Cells: Comparison between Endothelial Cells Isolated from Different Sources and Smooth Muscle Cells and Fibroblasts," Arteriosclerosis 10:437-48 (1990). Evidence for this conclusion comes from studies where the addition of plasma to a culture of subendothelial cells including vascular smooth muscle cells has been shown to cause rapid, massive coagulation. In contrast, clotting was inhibited when the experiment was repeated with endothelial cell cultures, again emphasizing the nonthrombogenic nature of endothelial cells. See for example, P. Colburn and V. Buonassisi, "Anti-clotting Activity of Endothelial Cell Cultures and Heparan Sulfate Proteoglycans," Biochem. Biophys. Res. Comm. 104:220-27 (1982); and I. Vlodavsky et al., "Platelet Interaction with the Extracellular Matrix Produced by Cultured Endothelial Cells: A Model to Study the Thrombogenicity of Isolated Subendothelial Basal Lamina," Thromb. Res. 28:179-91 (1982).
The widely accepted interpretation of these observations of natural vessel function is that the endothelial cell lining of the vasculature is responsible for antithrombotic behavior and that the subendothelial layers as well as the smooth muscle cells found beneath the endothelium are thrombogenic so that hemostasis will result in the event of vessel disruption.
Not surprisingly, in an effort to, adapt the antithrombotic function of the endothelium to synthetic surfaces, specifically vascular grafts, most prior art references that utilize biological elements are directed toward providing a surface that will support an endothelial cell lining. For example, U.S. Pat. Nos. 4,539,716 and 4,546,500 issued to Bell disclose a method of constructing a living tubular prosthesis using a collagen gel to which cells are added. The cells serve as a contractile agent and are specified to be fibroblast cells, smooth muscle cells, or platelets. For an artery replacement, these patents specify the use of endothelium as the most internal layer, smooth muscle cells as the medial layer and a third layer cast of collagen and fibroblast cells. The endothelial cells employed are of unspecified origin.
In addition, U.S. Pat. Nos. 4,804,381 and 4,804,382 issued to Turina et al. describe a synthetic arterial vessel made with a microporous or semipermeable membrane, lined on the luminal side with a continuous layer of living endothelial cells to provide the blood interface, and coated on the outside with layers of smooth muscle cells to increase the viability of the live cells on the lumen and to impart elasticity.
A number of other U.S. Patents including, for example, U.S. Pat. Nos. 4,883,755 to Carabasi et al., 4,960,423 to Smith, and 5,037,378 to Muller et al., also describe means by which living endothelial cell coverage of vascular interfaces can be accomplished to produce antithrombogenicity. These approaches include endothelial cell sodding, the use of elastin-derived peptides, and simple physical means of applying endothelial cells to graft surfaces, respectively.
A structure similar to that of Bell, above, is taught by H. Miwa et al., "Development of a Hierarchically Structured Hybrid Vascular Graft Biomimicking Natural Arteries," ASAIO Journal 39:M273-77 (1993). En this case, smooth muscle cells are layered over a DACRON.RTM. graft in an applied artificial matrix of collagen type I and dermatan sulfate glycosaminoglycan. A layer of endothelial cells is then grown on the artificial matrix to serve as the blood contact surface.
Another approach to endothelialization of vascular grafts is disclosed by X. Yue et al., "Smooth Muscle Cell Seeding in Biodegradable Grafts in Eats: A New Method to Enhance the Process of Arterial Wall Regeneration," Surgery 103:206-12 (1988). These authors employ pre-clotted microporous, biodegradable, polyurethane vascular grafts seeded with nonautologous rat smooth muscle cells prior to implantation for use as the replacement of the abdominal aortas of living rats. This study attempts to generate a "neomedia" to strengthen a structure that would otherwise be mechanically insufficient following the resorption of the biodegradable graft material. The smooth muscle cell layer provides a surface upon which the hosts natural endothelial cells could spontaneously regenerate and cover the graft surface.
In another study,, for example, A. Schneider et al. used corneal endothelial cells to produce extracellular matrix on ePTFE vascular grafts. (A. Schneider el al., "An Improved Method of Endothelial Seeding on Small Caliber Prosthetic Vascular Grafts Coated with Natural Extracellular Matrix," Clin. Mat. 13:51-55 (1993)) After production of an extracellular matrix, these original cells were then removed using Triton X-100 and NH:OH and the tubes were seeded again with bovine aortic endothelium. This approach showed that endothelium could be successfully grown on the extracellular matrix lining the ePTFE grafts, but no implant studies were performed, however.
In all of the above described prior art references, the efforts are directed at achieving a living endothelial cell lining to provide nonthrombogenic function. Despite the above described efforts, substantial deficiencies still exist. In small diameter applications or grafts used in sensitive areas, for example, even limited thrombus generation is a very serious concern. It should also be noted that the vivo performance of the above endothelial cell-coated grafts has been highly variable, without a clear demonstration of enhanced patency over existing grafts.
Accordingly, it is a primary purpose of the present invention to provide an improved blood contact surface that is less thrombogenic than existing artificial surfaces and structures.
It is another purpose of the present invention to provide an artificial blood contact surface that can be readily manufactured and used.
It is a further purpose of the present invention to employ a previously unrecognized mechanism governing the interaction between blood and natural blood vessel structures and to produce an improved blood contact device and methods for making and using them.
These and other purposes of the present invention will become evident from review of the following specification.